In wideband wireless communications through, for example, a wireless LAN system, such a problem has arisen that the quality of transmission is deteriorated due to multi-path fading or shadowing, especially in indoor applications. For this reason, it is required to develop a directive antenna mounted on a wireless apparatus capable of being controlled so that a primary beam radiated from it may advance toward any direction to maintain the quality of transmission at a moderate level even in a poor radio-wave propagation environment affected by multi-path fading or shadowing.
In addition, it is further required that an antenna, which is mounted on a notebook-PC type of terminal wireless apparatus for using on a desk or on a fixed type of wireless apparatus attached to a ceiling, has a planar structure because of these apparatuses' configurations. It is also required that the elevation angle of a primary beam tilts toward the horizontal direction from the vertical direction relative to the antenna plane.
As an example of a sector antenna providing such a radiation characteristic, a Yagi-Uda slot array planar multi-sector antenna has been disclosed in Journal of the Institute of Electronics, Information and Communication Engineers of Japan (IEICE) ((B) Vol. J85-B, No. 9, pp. 1633-1643, 2002). In the following paragraphs, the sector antenna is briefly described.
FIG. 1 is a plan view showing the configuration of a conventional sector antenna. As shown in the figure, each of slot arrays 11a to 11f has five-element slots vertically placed. The sector antenna has a configuration, in which the slot arrays 11a to 11f are placed in a radial pattern, drawing a circle. The primary beam radiated from each (for example, 11a alone) of the slot arrays, of which elevation angle θ tilts at any angle between 45° and 60° relative to the vertical plane, advances toward a horizontal plane. By placing these slot arrays at an interval of 60° relative to the horizontal plane (XY plane) and selectively feeding power to any of slot arrays 11a to 11f, the directivity of the primary beam can be switched among the sectors, each having an angle of 60° (360°÷6) The dimension of the sector antenna is 198 mm (equivalent to 3.3 wavelength) in diameter L17 and 30790 mm2 in area, assuming that the operating frequency of the antenna device is, for example, 5 GHz.
As another type of antenna, an end-open diamond-shape antenna, has been disclosed in the patent document JP-A No. 355030/1999 and Journal of the Institute of Electronics, Information and Communication Engineers of Japan (IEICE) ((B) Vol. J82-B, No. 10, pp. 1915 to 1922, 1999). FIG. 2 is a plan view showing the configuration of a conventional diamond-shape antenna. As shown in the figure, linear elements 21 and 22, each of which has a length equivalent to one wavelength of the operating frequency and has been bent at its center at a given angle, are placed so that they draw a diamond shape with a gap left between their apexes. In the case of this type of antenna, by feeding power at a feeding point 23, the primary beam advancing along a Z-axis perpendicular to the antenna plane (XY plane), may be obtained.
The conventional Yagi-Uda slot array planar multi-sector antenna aforementioned, however, has such a problem that it is difficult to mount on small size wireless apparatuses because the dimension of its plane incorporating six sectors is large and furthermore, the sectors need to be placed so that they may draw a circle.
Besides, the conventional end-open diamond-shape antenna aforementioned, of which primary beam advances in the direction perpendicular to the antenna plane, thereby does not tilt horizontally, has such a problem that it may not suitably mounted on the notebook-PC type of wireless terminal or the fixed wireless apparatus attached to the ceiling.